In recent years, with spread of broadband communication services at home, also in a mobile communication system of a cellular scheme, research and development are actively conducted for the purpose of providing high-capacity data communication services. However, radio frequencies which are finite resources are running short, and it is actively studied to utilize high frequency bands and realize a high transmission rate in order to realize high-capacity data transmission.
When high frequency radio bands are utilized, a high transmission rate can be expected in a short distance, but attenuation due to a transmission distance is significant. Therefore, when high frequency radio bands are utilized in a real system, for example, the coverage area of a base station becomes small, and it is necessary to provide more base stations. Providing base stations requires significant costs, and therefore it is strongly desired to suppress an increase in the number of base stations and provide the above communication services.
Therefore, in order to make it possible even for the mobile station located outside a cellular service area to communicate with the base station, it is studied to realize communication between the mobile station located outside the cellular service area and the base station by making another mobile station (relay station) located between this mobile station and the base station relay the communication.
For example, there is a technique for determining a relay route by comprehensively judging a plurality of channel quality—channel quality from the mobile station located outside the service area to relay stations and channel quality from relay stations to the base station—and determining at the base station a relay station which actually performs a relay out of a plurality of relay stations, and transmitting control signals to the plurality of relay stations indicating whether or not to perform a relay (for example, refer to Patent Documents 1 and 2).    Patent Document 1: Japanese Patent Application Laid-Open No. HEI7-321722    Patent Document 2: Japanese Patent Application Laid-Open No. 2004-254308